1. Technical Field
The present invention relates to a nonaqueous electrolyte secondary battery including a positive electrode having a layered lithium transition metal composite oxide as a positive electrode active material, a negative electrode having a negative electrode active material that can intercalate and deintercalate lithium, and a nonaqueous electrolyte having lithium ion conductivity.
2. Related Art
In recent years, as a battery for use in potable video cameras, cellular phones, portable electronic/communications devices, such as notebook personal computers, and the like, there has been commercialized a nonaqueous electrolyte secondary battery that includes a negative electrode active material made of a carbon material, alloy, or the like, that can intercalate and deintercalate lithium ions and a positive electrode active material made of a lithium-containing transition metal oxide, such as lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), or lithium nickel oxide (LiNiO2), and features its small size, light weight, and high voltage, as well as high capacity and rechargeability.
Of lithium-containing transition metal oxides used as a positive electrode active material for the nonaqueous electrolyte secondary battery described above, lithium nickel oxide (LiNiO2) features its high capacity, but has the drawbacks of poor safety and large overvoltage. Thus, it is inferior to lithium cobalt oxide. Lithium manganese oxide (LiMn2O4) is advantageously abundant in resources and low-cost, but has the shortcomings of low energy density and dissolution of manganese itself at high temperatures. Therefore, it is inferior to lithium cobalt oxide. For these reasons, lithium cobalt oxide (LiCoO2) is currently used as a lithium transition metal oxide in most cases.
In the meantime, lithium cobalt oxide (LiCoO2) is exposed to a potential of 4 V or more with respect to lithium. Therefore, when lithium cobalt oxide is used as a positive electrode active material for a nonaqueous electrolyte secondary battery, cobalt is eluted from the positive electrode each time the charge/discharge cycle is repeated. This undesirably deteriorates the positive electrode, reducing the capacity characteristics and load characteristics after each cycle is ended. Thus, there has been proposed a battery that uses LiNixCo1−xO2 obtained by improving lithium cobalt oxide (LiCoO2) and lithium nickel oxide (LiNiO2), as a positive electrode active material.
However, even if LiNixCo1−xO2 is used as a positive electrode active material, there remains a problem that the discharge capacity becomes smaller due to changes in crystal structure at the time of charge/discharge. Thus, a nonaqueous electrolyte secondary battery that can reduce changes in crystal structure to increase the discharge capacity as well as can improve thermal stability is proposed in Japanese Patent No. 3,244,314.
In the nonaqueous electrolyte secondary battery proposed in Japanese Patent No. 3,244,314, a lithium transition metal composite oxide (for example, Li1.0Mn0.1Ni0.45Co0.45O2.0) represented by LiaMbNicCOdOe (where M is at least one type of metal selected from the group consisting of Al, Mn, Sn, In, Fe, Cu, Mg, Ti, Zn, and Mo, 0<a<1.3, 0.02≦b≦0.5, 0.02≦d/c+d≦0.9, and 1.8<e<2.2, and b+c+d=1, 0.34<c) is used as a positive electrode active material.
Consequently, even if Li is extracted at the time of charge, the crystal structure is relatively stable. This prevents the crystal structure from collapsing even if charge/discharge is repeated, allowing reversible charge/discharge. Moreover, when M included in a positive electrode active material represented by LiaMbNicCOd Oe includes at least one type of metal selected from the group consisting of Cu and Fe, thermal stability of the battery in the presence of the electrolyte after charge can drastically be improved.
In addition, it is proposed in JP-A-2004-161526 to use a lithium transition metal composite oxide represented by Lic (NidMneCofMg) O2 (where M is at least one type of element selected from the group consisting of Fe, Cr, V, Ti, Cu, Al, Ga, Bi, Sn, Zn, Mg, Ge, Nb, Ta, Be, B, Ca, Sc, and Zr), 0.8≦c≦1.2, 0≦d≦1, 0≦e≦1, 0≦f≦1, 0≦g≦1, and d+e+f+g=1) as a positive electrode active material.
However, even if the positive electrode active material represented by LiaMbNicCodOe proposed in Japanese Patent No. 3,244,314 or the positive electrode active material represented by Lic (NidMneCofMg) O2 proposed in JP-A-2004-161526 described above is used, there occurs a problem that the I-V characteristic (I-V resistance: I-V resistance is obtained by measuring the voltage when a battery is charged or discharged for a given period at some currents, and then calculating the slope of the voltage to each current; the resistance serves as an index to see how many amperes of current can be passed through the battery,) is not improved. If the I-V characteristic (I-V resistance) is not improved, there is a problem that when such a battery is used as the power supply for an electric car, or the like, adequate output/input characteristics cannot be obtained.